A major part of mineral processing involves the separation of desirable minerals from impure ores within which the minerals are contained. The need to control iron is critical in the zinc, copper and nickel industry. Iron is in the form of iron sulfide, mainly pyrite (FeS2) and pyrrhotite (FeS). There are several options for iron management, such as flotation, hydrometallurgical and pyrometallurgical techniques. Further options in mineral processing are: liberation of locked particles by regrinding, feed dilution, reduction of circulating loads or washing the froth using flotation columns. The liberation of locked particles by grinding and/or regrinding is a technique to improve grade of zinc and iron rejection in concentrates. Another method is modifying the surface particle using collectors and the interaction of metal ions and precipitation on the mineral surfaces. Froth flotation is one technique employed to facilitate such separation. Prior to and/or during froth flotation the ore is ground and typically fed as an aqueous slurry to froth flotation cells. The chemistry of the slurry is adjusted at various stages such that certain minerals will selectively attach to air bubbles which rise upward through the slurry and are collected in froth near the top of a flotation cell, while other minerals and or parts such as gangue are hydrophilic and do not attach to the air bubble and sink due to gravity. Thereafter, selected minerals in the froth can be separated from different minerals and/or gangue in the cell.
During this process, the surfaces of specific mineral particles, which are in an aqueous state, are treated with chemicals called flotation reagents or collectors. The flotation reagents cause the desired mineral to be floated with a water-repellent coating that will easily adhere to an air bubble, which will raise the mineral through the slurry to the surface. Froth is also generated by vigorous agitation and/or stirring, and aeration of the slurry in the presence of a frothing agent. The desired mineral separated and collected during the froth flotation process may be either the froth product or the underflow product.
Other chemical agents can be added to the slurry to assist in the separation process, such as depressants or modifiers. The presence of depressants generally assists in selectivity and/or stop unwanted minerals from floating. In contrast, modifiers facilitate collection of desired minerals, and include several classes of chemicals such as activators, alkalinity regulators, and dispersants. Activators are used to make the mineral particle surface amenable to collector coatings. Alkalinity regulators are used to control and adjust pH, an important factor in many flotation separations, while dispersants work to control slimes which can interfere with selectivity and increase reagent consumption.
Flotation processes yields two main products, the concentrate and the tailing. The concentrate contains the metal value, while the tailing contains the gangue or undesired products or components that may also be considered to be wastes and/or less desirable products and/or desired products in a more impure state.
In general, entrainment and liberation are much easier and cheaper to implement than surface modification. However, it worth noting that the faster the removal of iron from the circuit, the cheaper the cost to treat iron. As a consequence, the flotation step is most preferable for dealing with the removal of iron.
Further, the process of reverse flotation in which pyrite is floated and sphalerite is depressed has been used in several mills to reject and control iron sulfides in the zinc final concentrate. Reverse flotation has been practiced on zinc concentrates but the cost of the heat and/or sulfur dioxide needed to de-activate sphalerite generally makes it impractical. Also, this process is environmentally unattractive.
Flotation has also been used to reduce entrainment of the less desirable iron sulfide with other more desirable metal sulfide particles. One such method is to modify the surface particle using collectors and the interaction of metal ions and precipitation on the mineral surfaces. Additionally, along with the collectors and/or frothers, ozone can be used to further facilitate separation of the particles by degrading and/or cleaning the coatings and residues left by reagents used in previous flotation stages. Additionally, longer ozonation time or ozone treatment oxidizes the mineral surface of more reactive substances, and it has been discovered that ozone oxidizes the surface of the iron sulfide minerals faster than the surface of the other more desired sulfide minerals.
Also, from an environmental viewpoint, iron sulfide is a source of sulfur dioxide at the smelter. Some countries have regulations requiring the reduction of sulfur dioxide emissions. Therefore, it is a further object to remove iron sulfide from the subject metals, thereby increasing the purity of the metals and decreasing the emission of sulfur dioxide during smelting.